(1). Race (Elf, Dwarf, Halfling) as a class? Yes or no?
Depends on my mood. Lately I've been more into the race-as-class thing. I've been running Labyrinth Lord, which is the first time in my life that I've done the race-as-class thing. But when I was a kid, I started with, and stuck with, AD&D and thus the separation of race and class.
(2). Do demi-humans have souls?
Depends on the setting, and how demi-human soullessness would fit into the setting. For instance, if dwarves were, perhaps, made from stone and had the spark of life but not the traditional sort of soul, that might, you know, be cool!
(3). Ascending or descending armor class?
Eh, these days, I'd have to say that ascending is my way of choice. Come to think of it, by extension, I like a good unified mechanic where all the rolls need to be high. Just easier for my aging brain to grok/implement at the table.
(4). Demi-human level limits?
No, I keep them going up. BUT, they of course have a bigger load of XP that they need to earn for those levels. I use the alternate demi-human leveling option from Swords & Wizardry.
(5). Should thief be a class?
Yes. I love the thief, I'm not afraid to admit it, I don't care who knows! ;-) Seriously, though, I am a believe that every character class can be a bit of a rogue, but thieves specialize in roguishness.
(6). Do characters get non-weapon skills?
Again, depends on the system. I'm a lover of Castles & Crusades, and their unified SIEGE Engine mechanic is a sort of catch-all for a skill system. But it doesn't have lists of actual, codified skills. If it makes sense for a character to have a skill, depending on class and background and other factors, then I allow them to have that "skill." I've also read through the Dragon Age RPG extensively of late, and that game has a fairly simple skill system (i.e. having a skill-called a "focus" in the game-allows you to at +2 to an attribute-based skill roll).
(7). Are magic-users more powerful than fighters (and, if yes, what level do they take the lead)?
Hmm, I guess. If they survive to high levels.
(8). Do you use alignment languages?
Uh, I don't even use alignment, let along alignment languages. No thanks to both!
(9). XP for gold, or XP for objectives (thieves disarming traps, etc...)?
XP for gold and all other treasure, as well as for killing monsters, completing "missions," roleplaying, etc. It's all in the mix.
(10). Which is the best edition; ODD, Holmes, Moldvay, Mentzer, Rules Cyclopedia, 1E ADD, 2E ADD, 3E ADD, 4E ADD, Next ?
The best system is the one that is loved and enjoyed by each individual roleplayer. Vive la difference!
Bonus Question: Unified XP level tables or individual XP level tables for each class?
I hate to repeat myself, but...it depends on the needs of the system!